A number of different systems are known for selectively applying power from a sinusoidal power source to a load. One example is U.S. Pat. No. 3,452,214 to Martin, entitled "Digital Wave Form Division For Power Control." Martin uses an oscillator to provide a timing signal comprising a series of pulses, which is counted by a pair of decade counters. The counters are initiated when a cyclic source waveform increases past zero volts. When the counters count a selected number of pulses, a gate provides a pulse to a silicon controlled rectifier, which in turn enables a transformer to supply electrical power from the sinusoidal power source to a load. The power delivered to the load is varied by changing the selected number of pulses.
A similar system is shown in U.S. Pat. No. 3,691,452 to Aguiar, entitled "Control of AC Power By A Logic Comparator." In Aguiar, a counter keeps track of increments of an oscillator, and digital logic gates determine when the output of the counter matches a digital input signal. No voltage is applied to the load until the output of the counter matches the digital input signal, whereupon silicon controlled rectifiers are triggered, thereby electrically connecting a periodic voltage source to a load, until the silicon controlled rectifiers are reset when the periodic voltage source next reaches zero volts.
Another example of a system for applying power to a load is shown in U.S. Pat. No. 4,260,947 to Massey, entitled "Phase Control Circuit For Regulating Power." In Massey, a sinusoidal source waveform is electrically connected to a load by a triac in response to a control trigger that occurs after pre-set counters reach their maximum values. The triac disconnects the source waveform from the load when a zero crossover detector determines that the source waveform has reached zero volts.
Another example is U.S. Pat. No. 4,352,045 to Widmayer, entitled "Energy Conservation System Using Current Control." Widmayer discloses a control system for electrical load devices such as fluorescent and incandescent lamps. In Widmayer, alternating current (A.C.) is supplied directly to a load; a transistor is used to control the magnitude of the current flowing through the load during the A.C. input voltage waveform. The transistor is turned full on when the A.C. input voltage waveform reaches zero volts, and can be turned off at any point during the input voltage half wave. A capacitor provides an alternate current path after the transistor is turned off, insuring that the load current is not abruptly terminated.
Still another power control circuit is embodied in U.S. Pat. No. 5,072,170 to Crane et al, entitled "Reverse Phase Angle Control Of A.C. Power Loads". Crane discloses both analog and digital arrangements. In the analog arrangement, an output driver is controlled by a pulse driver and an optoelectronic isolator, based on a square wave signal generated by comparing an analog input voltage to a ramp voltage. The output driver and a power supply provide current to turn on an output device. In the digital arrangement, an output device is electrically connected to a power supply. A digital logic circuit is used to turn on a load at zero crossover of a sine wave, and to turn off the load at a selected point within the sine wave.
Although the above-mentioned circuits are useful in some applications, these circuits are not capable of applying D.C. power to a load. One example of a power circuit that does facilitate application of D.C. power to a load is found in U.S. Pat. No. 4,821,166 to Albach, entitled "Power-Supply Circuit." In Albach, an A.C. source is connected to a power-storage capacitor during specific intervals controlled by a rectifier. A transistor switch, connected to the capacitor, is periodically actuated by a control circuit to discharge the capacitor into a load when the A.C. source is not charging the capacitor. Albach is thus said to separate the A.C. power supply and the switching circuitry, thereby preventing introduction of undesired interference such as high frequency voltages and currents on the A.C. source.
Another power circuit that applies D.C. power to a load is shown in U.S. Pat. No. 4,127,895 to Krueger, entitled "Charge-Transfer Voltage Converter." Krueger facilitates conversion of a high voltage A.C. or D.C. source to a D.C. voltage, to charge a load. After the power source charges a first capacitor, selective charging of a second capacitor by the first capacitor is controlled by transistor switches and an amplifier that compares the voltage of the second capacitor to a reference voltage. The second capacitor is directly connected to a load.
Another example of a power circuit that converts A.C. input power to D.C. power to charge a load is U.S. Pat. No. 3,372,326 to Stefanov, entitled "High-Efficiency Low Iron AC-to-Regulated D.C. Converter." In Stefanov, a switch is connected to an input transformer, and controlled by a control means. The output of the transformer is connected in parallel to a storage capacitor, a voltage regulator, and a load. The control means only closes the switch at selected times during the rising portion of the input sine wave. The closure of the switch is determined by the output of the voltage regulator and the input A.C. voltage.
Although the above-mentioned circuits are useful in some applications, they have a number of limitations. For example, some of these arrangements are limited since they require one or more transformers. Such circuits typically are more expensive and heavier than might be desired. Furthermore, the circuits that use transformers are not as efficient as might be needed, since they suffer from power losses caused by hysteresis, eddy currents, "I.sup.2 R", and other factors normally associated with transformers.
The above-mentioned arrangements that do not utilize transformers are lacking as well. In particular, these arrangements tend not to adequately isolate the load from the power source. For example, although Albach is said to prevent the introduction of interference into the power source, Albach still permits unwanted noise signals, fluctuations, spikes, and the like to be transmitted from the power source to the load. Furthermore, Albach cannot accommodate a non-cyclic source waveform such as a D.C. source.
In addition, many prior arrangements are not as useful as might be desired, since the application or removal of power to or from a load must be coordinated with time at which the power source voltage passes zero volts. For instance, some arrangements begin applying power to a load when the power source voltage passes zero volts, and stop applying power at a selected time. Other arrangements begin applying power to a load at a selected time, and stop applying power when the power source voltage passes zero volts. As a result, these arrangements do not provide a convenient means to deliver a precise quantity of energy to a load at a specific voltage, since either the "start time" or the "stop time" is fixed.
In view of the limitations of the known power supply circuits, an improved power supply is needed. Specifically, it would be advantageous to have a power supply capable of receiving either A.C. or D.C. input power, providing D.C. power to a load, and maintaining isolation between the power source and the load. Furthermore, it would be beneficial for such a system to avoid the use of a transformer. Additionally, it would be useful to selectively control the power supplied to a load, or intermittently provide power to a number of different loads.